1. Field of the Invention
This invention is in the field of signal processing, and is more specifically directed to acoustic drill string telemetry.
2. Description of the Related Art
The petroleum industry relies heavily on the operation of drilling into the earth, both on land and offshore, in the exploration for and production of petroleum products. Over the years, the more readily found and accessible petroleum reservoirs have of course been discovered and depleted first. As a result, the exploration and production operations must necessarily concentrate to a greater degree on less accessible and less readily discoverable reserves. In order to reach these locations, the depths of drilling have increased, the locations at which drilling takes place have become increasingly difficult and less accessible, and the drilling operations have necessarily become more complex. Accordingly, drilling operations in the search for and production of petroleum products have become more expensive, with this trend likely to continue in the future. Because of this increasing cost, the accuracy and efficiency of the drilling operation is becoming even more important.
The success and efficiency of the drilling operation depends to a large degree on the quantity and quality of information that the drilling operator has about the sub-surface structure into which the drilling is taking place, and also about parameters concerning the operation of the drill bit as it proceeds into the earth. Many techniques for acquisition and communication of such information have been tried and used in the industry.
A system which utilizes the drill string as a medium for the transmission of data is referred to as acoustic telemetry or stress wave telemetry. Acoustic telemetry systems are known in the art. For example U.S. Pat. No. 5,477,505 to Drumheller and U.S. Pat. No. 5,303,203 to Kingman describe such systems. The typical system includes transmitters, such as solenoids, eccentric motors, and piezoelectric transducers, which intentionally vibrate the drill string in a manner corresponding to the desired data. These data may include information concerning drilling parameters and formation parameters. In the case of stress wave telemetry the desired information is obscured by undesirable bit and drilling noise that is also transmitted through the drill string.
It has been discovered that vibrations, whether from the drill bit itself or intentionally generated by transmitters, are not communicated through the drill string in an ideal manner, due to the non-ideal response of the drill string to such vibrations. Conventional drill strings, which consist of a number of lengths of drill pipe joined by pipe joints, inherently have frequency domain stopbands that attenuate acoustical signals at the stopband frequencies. This frequency-dependent attenuation can severely distort some signals. Other factors also distort the vibrations communicated along a drill string from downhole to the surface. Such factors include noise generated by the drilling fluid, or mud, which is conventionally pumped through the drill string at relatively high pressures. This high pressure flow of fluid causes significant vibrations in the drill string. Other devices in the drilling operation, such as bearings in the swivels at the top of the drill string, the rattling of chains which turn the kelly bushing, or the motor in a top drive drilling arrangement, and the slap of the casing against the drill string or well bore, also generate significant acoustical vibrations which are received by and transmitted along the drill string. These vibrations are superimposed upon the desired data signal, and will accordingly be detected at the top of the drill string by such detectors as are attempting to detect the data signal transmitted from the downhole location.
Considering the vibrations generated by a transmitter as “signal” and the vibrations generated by the drill bit and the other vibrations caused by drilling mud flow and the mechanical sources discussed in the prior paragraph as “noise”, it has been found that the amplitude of the noise can be substantially greater than the signal amplitude. Noise at this level not only clouds the analysis of the information, but indeed drowns out the information itself.
Vibration-state inference techniques have been described to determine downhole force and displacement at a position close to the bit from similar measurements at a second location in the drillstring, (see SPE 74718, Macpherson, et al., “Application and Analysis of Simultaneous Near Bit and Surface Dynamics Measurements”, SPE Drilling and Completions, Society of Petroleum Engineers, December 2001). However, there is no suggestion therein of using such a technique for purposes of acoustic telemetry in a drillstring.
The methods of the present invention overcome the foregoing disadvantages of the prior art by providing a technique for removing a portion of the surface generated noise thereby improving the signal to noise ratio of acoustic signals transmitted along a drill string.